The present invention relates to a process for the preparation of meta-toluene-diisocyanate by phosgenation of meta-toluenediamine in the gas phase.
Isocyanates are prepared in large amounts and serve chiefly as starting materials for the preparation of polyurethanes. They are usually prepared by reaction of the corresponding amine with phosgene. One possibility for the preparation of isocyanates is the reaction of the amine with the phosgene in the gas phase. In this process which is conventionally called gas phase phosgenation, the reaction conditions are chosen so that at least the reaction components amine, isocyanate and phosgene, but preferably all of the educts, products and reaction intermediate products, are gaseous under the conditions chosen. Among the advantages of gas phase phosgenation are, inter alia, a reduced phosgene hold-up, the avoidance of intermediate products which are difficult to phosgenate and increased reaction yields. The present invention relates exclusively to gas phase phosgenation.
Various processes for the preparation of diisocyanates by reaction of a diamine with phosgene in the gas phase are known from the prior art.
Specifically, phosgenation of aliphatic diamines in the gas phase has often been described. Thus, EP 289 840 B1 discloses a process for the preparation of diisocyanates by phosgenation of the corresponding diamine(s) in the gas phase, in which the vaporous diamine(s), optionally diluted with an inert gas or with the vapor of an inert solvent, and phosgene are heated separately to temperatures of from 200° C. to 600° C. and are reacted with one another continuously in a cylindrical reaction space while maintaining a turbulent flow. The gas mixture leaving the reaction space is passed through an inert solvent which is kept at a temperature above the decomposition temperature of the carbamic acid chloride corresponding to the diamine. The diisocyanate thereby dissolving in the inert solvent is subjected to working up by distillation.
The reaction of aromatic diamines with phosgene in the gas phase to give the corresponding diisocyanates is also described in the literature.
EP 593 334 B1 discloses a process for the preparation of aromatic diisocyanates in the gas phase in which a tube reactor is used. Mixing of the educts is achieved in this process by narrowing the walls of the tube reactor. The reaction is carried out in the temperature range of from 250 to 500° C. However, the process is problematic because the mixing of the educt streams solely by narrowing the tube wall functions poorly compared with the use of a proper mixing device. Poor mixing conventionally leads to an undesirably high formation of solids.
There have been many attempts to minimize this formation of solids which is particularly encountered in the reaction of aromatic diamines with phosgene in the gas phase, to make it possible to phosgenate aromatic diamines in the gas phase on a large industrial scale. In this context, the improvements in the process for the large-scale industrial phosgenation of aromatic amines in the gas phase focus on improving the mixing of the educt streams and equalizing the flow in the gas phase reactor, which lead to a prolonged service life of the gas phase reactor.
EP 570 799 B1 discloses a process for the preparation of aromatic diisocyanates, which is characterized in that the reaction of the associated diamine with the phosgene is carried out in a tube reactor above the boiling temperature of the diamine within an average residence time of from 0.5 to 5 seconds, and in which the average deviation from the average residence time is less than 6%. According to the teaching of EP 570 799 B1, both residence times which are too long and those which are too short lead to undesirable formation of solids, so that an equalizing of the flow in the reaction space is necessary, and above all back-mixing of the components in the reaction space is to be ruled out.
Measures for equalizing the flow conditions are likewise the subject matter of EP 1 362 847 B1. EP 1 362 847 B1 discloses a process for the preparation of aromatic diisocyanates in the gas phase in a tube reactor. In this process, control of the flow (e.g., equalizing and centering of the educt streams), and reduction in temperature variation with respect to time and an asymmetry in the temperature distribution make it possible, according to the teaching of EP 1 362 847 B1, to avoid caking and blockages in the reactor and therefore to a shortening of the service life of the reactors.
According to the teaching of EP 1 449 826 A1, the reaction of the aromatic diamine with phosgene in the gas phase the reaction of the phosgene with the diamine to give the diisocyanate competes with the secondary reaction of the diamine with the diisocyanate to give the corresponding urea oligomer. EP 1 449 826 A1 teaches that an improved mixing of the educts phosgene and diamine while simultaneously avoiding back-flow in the tube reactor increases the selectivity of the diisocyanate formation and reduces the formation of urea. As a result, according to the teaching of EP 1 449 826 A1, the amount of condensation product in the tubular reactor, which, because they are deposited on the reactor wall, lead to a reduction in the size of the free tube cross-section and to a gradual increase in pressure in the reactor and in the end determine the service life of the process, can be reduced. Apparatus solutions for improved mixing of the educts are likewise disclosed in EP 1 526 129 A1, DE 103 59 627 A1 and WO 2007/028 715 A; EP 1 526 129 A1 (flow measures for generating spin); DE 103 59 627 A1 (concentrically arranged annular nozzles with single); WO 2007/028 715 A (multiple amine feed); and EP 1 449 826 A1 (several amine nozzles arranged parallel to the axis of a tube reactor).
Nevertheless, not only the physical reaction conditions but likewise the properties of the aromatic diamines employed in the reaction with phosgene in the gas phase were the subject matter of the processes disclosed.
According to WO 2008/071 564 A, amines which are to be reacted in a gas phase phosgenation to give the corresponding isocyanates must meet certain requirements. Specifically, those amines which decompose to the extent of no more than 2 mol %, more preferably no more than 1 mol % and most preferably no more than 0.5 mol % under the reaction conditions prevailing in the gas phase reactor are suitable. According to the teaching of WO 2008/071 564 A, these are aliphatic or cyclic amines. According to WO 2008/071 564 A, aromatic amines can also be used if they can be converted into the gas phase without significant decomposition. WO 2008/071 564 A discloses that aromatic amines which are preferably suitable are toluenediamine (TDA), as the 2,4 or 2,6 isomer or as a mixture thereof, for example as an 80:20 to 65:35 (mol/mol) mixture; diaminobenzene; 2,6-xylidine; naphthyldiamine; and 2,4′- or 4,4′-methylene(diphenylamine) (MDA) and isomer mixtures thereof. However, instructions as to how the aromatic diamines which are described as preferably suitable can be converted into the gas phase without significant decomposition are not found in WO 2008/071 564 A.
EP 1 935 876 A1 also recommends the use of aromatic amines which can preferably be converted into the gas phase without decomposition. This specification discloses a process for the preparation of isocyanates in the gas phase, in which the reaction space has neither heating surfaces that can give rise to exposure to heat with the consequence of secondary reactions, such as isocyanurate or carbodiimide formation, nor cooling surfaces that can give rise to condensation and cause deposits.
EP 1 754 698 A1 discloses a specific vaporization technique which takes into account the exposure of the amine(s) employed in a gas phase phosgenation to heat. According to the teaching of EP 1 754 698 A1, the deposits observed in the reactor for reaction of the amine(s) with phosgene are caused by decomposition, during the reaction, of the amine(s) employed. This disclosure also teaches that long dwell times in the vaporization and superheating lead, specifically if aliphatic amine(s) are employed, to a partial decomposition of the amine(s) with ammonia being split off. This partial decomposition with splitting off of ammonia during the vaporization observed if aliphatic amities are employed not only reduces the yield, but results in the formation of deposits of ammonium chloride in the downstream pipelines and apparatus during the subsequent phosgenation reaction. The equipment must then be cleaned relatively frequently, resulting in production losses. EP 1 754 698 A1 states that these disadvantages occur in particular with the tube bundle heat exchangers, plate heat exchangers or falling film evaporators conventionally employed for the vaporization and superheating of the amines. As a technical solution, this disclosure teaches that the splitting off of ammonia during the vaporization is suppressed by employing specific milli- or micro-heat exchangers for the vaporization and superheating of the aliphatic amines. In the process disclosed, the amines are vaporized completely in the evaporator, so that circulation streams through the apparatus are eliminated, so that the amine flows through the apparatus only once.
The very small channels are a disadvantage of the micro heat exchangers disclosed in EP 1 754 698 A1. Very small amounts of solids, which are always present in industrial processes, already lead to a blockage and therefore reduce the service life of the evaporator. It is also a disadvantage that the amine to be vaporized should not contain any other not vaporizable constituents, because these other not vaporizable constituents would be deposited as a solid residue on the evaporator surface and therefore impair the heat transfer and finally lead to blocking of the evaporator. However, the provision of amines in the required quality is very involved and expensive in the industrial process. The service life of the reactor is improved by the teaching of the specification, but the service life of the evaporator system is impaired so significantly that the total service life of the production installation is not advantageously improved.
Minimizing exposure of the amines to heat during their vaporization for reaction with phosgene in the gas phase is likewise the subject matter of EP 1 935 876 A1. EP 1 935 876 A1 teaches that before the reaction with phosgene, the amines as a rule are vaporized and heated to 200° C. to 600° C. and are optionally fed to the reaction space in a form diluted with an inert gas (e.g., N2, He or Ar), or with the vapors of an inert solvent (e.g., aromatic hydrocarbons, optionally with halogen substitution, such as chlorobenzene or o-dichlorobenzene). This disclosure teaches that the vaporization of the starting amine(s) can be carried out in any of the known vaporization apparatuses. Vaporization systems which are described as being preferred are those in which a small work content is led with a high circulating output over a falling film evaporator. Minimization of the exposure of the starting amine(s) to heat in the vaporization process is optionally assisted by feeding in inert gas and/or vapors of an inert solvent.